Sol-gel processing of metallic oxide glasses has recently become the subject of intense research because of its interesting applications. When compared to more conventional processing, the inherent advantages of sol-gel processing include chemical homogeneity on a molecular scale, high purity, and the ability to conveniently shape articles by the use of low temperature casting methods. The term "sol-gel" is an abbreviation of the phrase "solution-gelation". In general reference, the term sol refers to a mixture of solid colloidal particles in a liquid solvent. Brownian motion keeps the colloidal sols from settling, while surface charges aid in the dispersion of the particles within the solvent. However, in the glass field, sol-gel has a broader meaning because it includes the use of organometallics such as metallic alkoxides which are partially hydrolyzed and then polymerized into a uniform gel. During the processing steps, there are major distinctions between polymerized alkoxides and colloidal sols.
While colloidal sols are prepared by precipitation and peptization, polymerized alkoxides are partially hydrolyzed, EQU M(OR).sub.x +yH.sub.2 O.fwdarw.M(OR).sub.x-y (OH).sub.y +yROH
wherein R represents an alcohol group. The partially hydrolyzed species link to form M--O--M bonds by a polymerization or condensation reaction. Once polymerized, the solvent is evaporated and the body is then processed for use. These alkoxide processes are able to produce transparent glass bodies if the pore sizes are less than the wavelength of light. These transparent bodies may be weakly crystalline due to very small particle size.
Applications for the glass bodies made by alkoxide processing include nuclear fuel preparation, radioactive waste immobilization, preparation of ceramic powders, preparation of bulk glasses, glasses for coatings, optical fibers and the fabrication of glass-ceramics. High purity bulk glasses formed into plates have many possible applications, including the semiconductor industry. For example, silica glass bodies made from metal alkoxides are used as substrates for thin film transistors and integrated circuit masks.
Silica glass is one of the best known glass materials for semiconductor devices because it has good thermal stability, chemical durability and optical transparency over a wide range of wavelengths. A specific advantage of silica glass is the ease with which the chemical composition may be controlled. Alkali-free glass is especially useful for semiconductor substrates because alkali ions within the body of conventional soda lime plate glass substrates diffuse into the semiconductor materials deposited on the body of glass and change the doping gradient of the semiconductor material. This change has a negative effect on the efficiency, speed, and overall optical and chemical characteristics of the semiconductor device.
Sol-gel glass bodies have particular utility in the fabrication of amorphous silicon photovoltaic cells because they require substantially sodium free substrates are required to maintain peak efficiency of the conversion of sunlight into electricity. Sodium ions migrating from a conventional soda lime plate glass substrate into the semiconductor body material of a photovoltaic cell changes the doping profile of the semiconductor and consequently lowers the efficiency. Because each layer deposited in an amorphous silicon photovoltaic cell interacts with the anothers, compatibility is crucial. This special application requires a very smooth surface to enable effective conformal deposition of all layers. Conventional sol-gel processing yields glass bodies which are not sufficiently smooth for semiconductor applications.
Currently, silica glass bodies are formed by sol-gel processing. Gel glasses are formed from siliceous solids emulsified in a liquid solvent which then undergo a gelation solidification step, slowly and continuously increasing the viscosity of the gel. The solvent is slowly evaporated from the body, forming a block of silica glass having a porous structure. The silica glass is then sintered to collapse the pores to form a non-porous body. This sintered body is not sufficiently smooth on a microscopic level to be used for semiconductor applications because the collapsed pores cause textured surface defects. Although the textured surface defects are no longer jagged peaks after sintering, the height of the texture is still too great to receive depositions of layers of semiconductor material. Typically, to achieve the desired smoothness, the sintered plate is then polished and ground, which is an expensive and inefficient method.
U.S. Pat. No. 4,323,381 issued on Apr. 6, 1982 to Matsuyama et al., discloses a process for producing a mother rod useful for the production of an optical fiber which comprises hydrolyzing a silicon alkoxide to prepare a gel in the desired shape, and drawing and sintering the gel to produce a block made of silica glass. The dried gel is gently raised in temperature and then sinterd at the collapse temperature, but below the melting temperature of the dry gel.
Japanese Pat. No. 85-101,966/17 issued to Suwa Seikosha KK on Mar. 14, 1985 discloses a manufacture for a plate made by hydrolyzing methyl silicate to form silica sol, followed by gelation, drying and sintering. The stated improvement comprises the charging of silica into a vessel, rotating the vessel while situated at a preferred distance from the rotating sinter to be gelled, followed by drying and sintering. The advantage is a silica glass substrate of high quality which is produced simply by centrifugal force.
Japanese Patent No. 85-101,967/17 issued on Mar. 14, 1985 to Suwa Seikosha KK discloses a flat, plate-like silica glass manufactured from at least a metal alkoxide under low temperatures by the sol-gel process. The improvement comprises putting plate-like dry gel on the surface of a heat resisting board which is placed in parallel with the surface of the floor, followed by subjecting the plate to sintering. The sintering treatment is carried out by slowly heating the dry gel up to 1200 degrees Celsius. Disclosed is a product having flatness, as the plate-like gel is treated to avoid warping or bending. Plate-like silica glass obtained through this method is suitable for use as a substrate for thin film transistors or integrated circuit masks.
Polymerized metal alkoxides are acid catalyzed to form transparent gels which are uniform polymers; see, for example, Sol-Gel Processing of Ceramics and Glass, D. W. Johnson, Jr., American Ceramics Society Bulletin Vol. 64, Dec. 19, 1985, pages 1597-1602.
Tetraethylorthosilicate (TEOS), the ethoxide of silicon, is mixed with water and an acid catalyst to increase the rate of hydrolyzation to form a transparent gel. After drying, the silica body is sintered and polished by a spinning process. The body is then annealed at temperatures from 200.degree. to 400.degree. C.